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Bio-Mat 101

To better understand Bio-Mat, let’s review what happens when septic effluent is introduced into soil using gravity distribution.

Effluent flows from the septic tank into the soil treatment trench through a number of holes that are ½ inch in diameter in the distribution pipe. Then through the distribution media (rock in our case) to the soil surface. When the effluent reaches the soil a condition called biomat is created. The biomat is formed by anaerobic bacteria in the effluent and any finely sized organic suspended solids carried over from the septic tank.

The bacteria secretes a sticky substance around the outside soil and rock particles. The biomat develops along the trench bottom and ponds the effluent in the trench. As the liquid rises, biomat develops along the sidewalls.

The main result of biomat formation is dramatic slowing of the infiltration rate of the effluent into the soil, creating unsaturated flow conditions. This is ideal for growth of aerobic bacteria and other soil organisms that help with treatment of pathogens and other contaminants. THIS IS GOOD NEWS. This explains why the biomat--- when properly managed---is a necessary component of soil treatment capabilities. In addition the biomat reaches equilibrium. If effluent quality is maintained, the biomat will have the same thickness and permeability overtime.

This condition is referred to as the Long Term Acceptance Rate, or LTAR. The LTAR is related to soil texture class, soil structure, and consistency to predict the loading rates in soil where the biomat is fully developed. Research on these relationships has been conducted since the early 1970’s and in fact, the condition was recognized as early as the 1950’s.

Sandy Soils are Different

These relationships are reflected and used in almost all current codes dictating the design and installation of onsite treatment systems. Sandy soils are the one exception.

Since the sandy soils particle size is larger (0.025-2.0mm) the size of the pores are also larger, allowing effluent to move more rapidly into and through the sand, often without forming a biomat. (Remember with rock it slows down the movement of effluent). This rapid movement does not allow time for treatment. So in the early ‘70’s and again as recently as 2006, column studies were conducted looking at virus removal in sands under different loading rates.

These studies showed good virus removal in 2 feet of sand if the loading rate does not exceed 1.2 gallons per square foot/day. This has been confirmed by the state of Florida. This is the loading number for sands found in most of our codes today.

One unexpected result of this study showed a thin biomat formed in the soil and this biomat was very effective at reducing the flow rate into the soil. Over several years of study, the biomat showed an acceptance rate of 0.6-gallons/square foot/day. This was confirmed over the next several years and the loading rate was incorporated into the state code.

Remember we are talking about a rock media in the drainfield, which Dixie Septic Tank Inc. uses. Our competition does not use rock or any media other than sand.

Another interesting note on how sandy soils react to septic tank effluent: If you conduct a percolation test, the rate would be in the range of 30 seconds to 3 minutes per inch: so you would not distinguish these soils from other types of sands on the basis of percolation rates.

The Bottom Line

So what about the treatment approaches in these soils? For the coarse and medium sandy soils, the only way to ensure the loading rate doesn’t exceed 1.2 gallons/square foot/day is to spread the effluent out evenly over the entire soil treatment area. “HALLELUJAH” This is exactly what Dixie Septic Tank does with our “Equal Distribution™” system, using our Patented Pipe, and using recycled concrete as the drainfield media.

So if you are looking for a great system that will last a long, long time, give us a call, we are more than willing to help. (386) 738-3030.

Septic System FAQs

We are pleased to offer a few articles we have written over the past several years. We hope they might offer tips and answers to questions or issues you may have in regards to septic tanks, septic systems and drain fields. Please contact us directly with specific questions for a personal consultation.

Septic System FAQs
Smart Septic Systems
Going Green
Aggregate Rock System
Equal Distribution™
Pre-Contract Checklist
Septic Systems 101
Septic System Maintenance
Bio-Mat 101
Flushing Medication
Facts and Folklore
Pipe and Rock System
Septic Tank Installation
What is OSTDS?
Fat, Oil and Grease
Septic Industry Problems?
Sewer or Septic System?
Time to Take the Gloves Off!
Failing Septic Systems
Eye Opening Definitions
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Dixie Septic Tank, Inc.

335 N. Boundary Avenue
Deland, Florida 32720

Phone: (386) 738-3030
Fax: (386) 740-7666
Lic# SRO991327
Contact Dixie Septic Tank

Home Page Failing Septic Systems Septic System Maintenance What is OSTDS?
Septic Services Going Green Bio-Mat 101 Fat, Oil and Grease
Smart Septic Systems Aggregate Rock System Flushing Medication Septic Industry Problems?
Concrete Septic Tanks Equal Distribution™ Facts and Folklore Sewer or Septic System?
Septic System FAQs Pre-Contract Checklist Pipe and Rock System Time to Take the Gloves Off!
Contact Us Septic Systems 101 Septic Tank Installation Failing Septic Systems
Septic Tank Cleaning Eye Opening Definitions
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